In this thesis, the optical response of quantum dot molecules without and with the presence of metal nanoparticles has been studied. Using the density matrix formalism, the system's behavior is analyzed, and the governing equations of the quantum system are solved in the steady state. By solving the dynamic equations of the system, the linear and nonlinear optical responses have been investigated by calculating the first- and third-order susceptibilities, and the two-photon absorption cross section of quantum dot molecules has been determined. In addition, the phenomenon of induced tra  arency resulting from electron (hole) tunneling between quantum dots in quantum dot molecules has been studied, and the effect of physical parameters such as the tunneling intensity, the energy difference between the energy levels, and the distance between the quantum dot and the metal nanoparticle in a quantum dot molecule system has been investigated. The results obtained can be useful in improving the control of optical properties and designing nanophotonic and optoelectronic structures.

This study investigates the physical properties of the two-dimensional Si?BN structure and evaluates its potential use as an absorber layer in solar cells. The primary goal was to optimize this material by introducing semiconducting properties through the opening of its bandgap. In its pristine state, Si?BN exhibits a zero bandgap and metallic behavior. To modify this property, two approaches were employed: (1) doping the pristine structure with aluminum atoms (doping method) and (2) combining Si?BN with a thin layer of gallium nitride (GaN) to form a bilayer heterogeneous structure.The calculations were performed within the framework of density functional theory (DFT). The WIEN2K code was used for the computations, and the generalized gradient approximation (GGA) was employed for the exchange-correlation potential, providing high accuracy in determining the system’s energies and properties. The results showed that in the aluminum-doped state, the bandgap did not open, and the metallic behavior of the material remained unchanged. In the heterogeneous structure, a very small bandgap of 0.118 eV was observed, but the material still did not exhibit semiconducting properties. Therefore, this study was unable to optimize Si?BN for use as an absorber layer in solar cells.However, due to its excellent conductivity and high carrier mobility, Si?BN (in both its pristine and modified states) can be utilized as an electrode or tra  ort layer in solar cells.